In known apparatuses of this type, some are of the atmospheric air type such as a VENTURI system or of the type having air pulsed by a fan. These devices have the disadvantage of using air as an oxidizer, hence a considerable loss of energy due to the fact that oxygen alone participates in the combustion reaction, whereas nitrogen, a neutral gas, absorbs a considerable part of the power energy during the endothermic process of the chemical reaction and is evacuated in the fumes or, rather, combustion by-product gas.